How to print a 5x7 photo card

The best way I can think of is to set up a paper size that is 1/2 the size of the paper you are printing onto. The photo will then be printed centred on the first half of the paper as it comes through the printer.

Since photos can only select paper sizes set up in your printer driver, you'll have to create a custom paper size there first. How you do this will depend on your printer software.

EDIT: just tried to do it here - it is not easy to find in my Epson printer software. I had to open a document in pages, choose page setup, then in the paper size selector I could choose custom and set up a custom size there, which was then available in photos. See screenshots (it is ridiculous that it doesn't seem possible to do this from the printer settings in system preferences, or generally from the printer setup in any app)

It might be best to export the image (or use the media browser) to Pages and use that to locate the image. You will have full control over placement and printing. And can add a greeting!